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The excitation functions eCL(E, T,) for forming chemiluminescent (CL) products in the (Ba + N20) reaction is found to depend sensitively on N20 vibrational temperature T,. In a second experiment thermal rate constants for CL production kcL, and for Ba beam attenuation kT, were measured as functions of N 2 0 temperature, and activation energies were obtained. Analysis shows that u2bending acts as promoting mode, and participation by uI (N-0 stretch) is highly likely. Promotion by v2 is consistent with the initial formation of an ion pair Ba+N20-. Rearrangement of this intermediate to BaO*(A' 'n) products, believed to be the CL emitter, is adiabatically allowed.Specificity of energy consumption is traditionally discussed in the context of vibrot-el-trans reactant excitation in direct A + BC reactions.' A further aspect enters the picture with polyatomic molecules: The 3N -6(5) vibrational degrees of freedom will generally show mode-specificity. Although little is known at present about these matters, simple symmetry considerations alone, i.e., the degree to which a given normal vibration projects onto the reaction coordinate, provide a starting point for predicting the efficacy of that mode.The present chemiluminescent (CL) atom-triatom reaction Ba + N20: --+ BaO* + N2(1) will be shown to differ in an interesting way from such a naive expectation. This sheds a new light on its reaction dynamics. The Ba + N 2 0 system has been studied extensively in recent years, with particular emphasis on problems of energy disposal and the nature and effects of the so-called " reservoir states ".2 In this paper we report energy consumption measurements by two complementary methods :(1) A crossed supersonic/thermal beam experiment alllows us to measure the CL excitation function eCL(j?, T,) over a range of vibrational temperatures T,. Supersonic beams of a constant nominal energy3 are generated by changing the He/N20 seeding ratio as well as the nozzle temperature To. Since the vibrational distribution remains essentially unrelaxed at To, this allows one to vary T, N To independently from J?. A pronounced vibrational enhancement of ecL(E7 T,) was found, in agreement with an early report on this The lowest collision energy achieved in this experiment was 9 kJ mol-'.(2) In order to cover the low energy region, thermal rate constants kcL(TN20)

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